Carbonate spherules and botryoids as lake floor cements in the East Kirkton Limestone of West Lothian, Scotland

Abstract

ABSTRACTThe East Kirkton Limestone is typically a carbonate/organic laminite characterised at many levels by abundant radial-fibrous calcite (RFC) spherules and by less common larger stromatolite-like accretions of laminated botryoidal RFC. The spherules are mostly c. 1 mm in diameter and have cyanophyte and chlorophyte inclusions. Some spherules enclose parallel bundles of complete cyanophyte fibres and probably grew within a living cyanophyte mat. The botryoidal accretions were commonly seeded upon wood and other exposed organic remains such as bone, and they completely enclose twigs and branches where these were held above the sediment surface. Botryoidal accretions commonly contain the remains of a benthos of cyanophytes, chlorophytes and ostracods.Both types of calcite have carbon and oxygen stable isotope values similar to those of known fresh-water precipitates. Their stable isotope and trace element geochemistries are consistent with precipitation on the floor of a tropical fresh-water lake within a volcanic setting, but removed from the influence of any hot-spring activity. Spherules and botroids are mineralogically closely similar and, whilst precipitation may have been biogenically mediated, they are regarded as passive lake floor cements.The carbonate laminae are dominated by rhombohedral calcite. Many of these laminae may have originated as calcite suspensoids which settled to the lake floor during relatively brief precipitation events, blanketing the normally richly organic substrate, smothering the cyanophyte mats, and leading to the preservation of individual organic laminae. Crystals later became enlarged and intergrown within the sediment, but this occurred early and prior to significant compaction, because detail of fragile and degradable organic constituents is commonly preserved. The likely source of the carbonate is through leaching of the local basic volcanic terrain. The precipitation of two types of calcite implies regular fluctuations in the chemistry of the lake waters, or in the factors controlling precipitation, which may have been a biogenic and/or seasonal effect.